Unhexquadium
Unhexquadium, Uhq, is the temporary name for element 164. It is expected to be a transient element, one without long-lived isotopes or long-lived precoursers. Uhq may form during neutron star mergers. NUCLEAR PROPERTIES A proton shell closure is predicted to occur at Z = 164. At least one model exists predicting the half-lives and decay modes for nuclides up to Z = 175 and N = 333(1), which includes isotopes Uhq 497 and lighter. It is helpful to view p 18 of Ref. 1, which maps predicted half-lives of nuclides in this region, and p 15, which maps principal decay modes. These maps are the result of large extrapolations from what can be tested, though. Half-lives are accurate only to within three orders of magnitude and decay modes give no information about competing minor decay modes. Results given in this article should be regarded as tentative. Ref. 1 predicts a band of nuclides ranging from Uhq 449 to Uhq 473. In addition, Uhq 435 is predicted to survive long enough to be a nuclide. Given adjacent light nuclides down to Uht 438 and Uhb 435, Uhq 435 is more likely to represent the actual beginning of a band ranging from Uhq 435 to Uhq 473 than for it to be an artifact. As neutron count increases in this band, half-life increases, reaching a maximum exceeding a second, although it is unlikely that any isotopes in this band have half-lives exceeding 1000 sec. (Beta-stable nuclides are identified via black boxes, which means both decay mode and half-life have to be estimated from properties of adjacent nuclides which are not beta-stable.) Principal decay mode shifts from fission to alpha emission in this band. This pattern is expected, given the predicted neutron shell closure at N = 308. However, there is also a band of alpha-decaying isotopes with millisecond-scale half-lives predicted to lie between Uhq 474 and Uhq 484, a zone which should be strongly destabilized by the N = 308 closure. A neutron shell closure has also been predicted to occur at N = 318(2) and a proton shell closure has been predicted at Z = 154(3). The long lives reported for Uhq 474 to Uhq 484 may indicate effects of those two closures. Between Uhq 484 and Uhq 497, the predicted short-lived, fission-decaying isotopes and nuclear drops too short-lived to be nuclei are what is expected. Beyond Uhq 497, predicting nuclear properties is largely guesswork. At least two sets of predictions exist for location of the neutron dripline up to Z = 175(3),(4). These two indicate that the dripline occurs between Uhq 562 and Uhq 593. Neutron shell closures are also predicted to occur at N = 370(3) and 406(5). Ref. 5 also includes predicted structural correction energies provided by shell closures at N = 406 and Z = 164. Applying the Z = 164 correction at full strength predicts that Uhq 461 through Uhb 495 have fission half-lives exceeding 0.001 sec, which is inconsistent with the much higher-quality results of Ref. 1. Multiplying the Z = 164 corrective energy of Ref. 5 by 0.7 makes the two sets of data consistent with each other. With this adjustment, beta-decay can be expected to predominate in isotopes between Uhq 495 and Uhb 593, with a significant beta-decay branch occurring as low as Uhq 490. (Since the pattern of nuclear properties given in Ref. 1 hints that nuclides in this region either were not modeled or that the model begins to break down in this region, this is not necessarily a contradiction.) The band from Uhq 495 to Uhq 562 is particularly important, since it lies below the lower neutron dripline prediction, strongly implying that (at least the heavier of) these isotopes will beta decay. Although it cannot be quantified, the N = 370 closure should enhance stability of isotopes in a band which may extend as far as Uhq 520 to Uhq 540. FORMATION Polar jets emanating from young neutron stars and black holes function like giant, sloppy particle accelerators. It is possible for fusion or multinucleon transfer reactions to produce all isotopes of Uhq, but only in atoms-per star quantities. This section addresses possible isotopes which can form in quantity. Material originally found 800-1000 m beneath the surface is expected to be ejected from a neutron star when it disintegrates during a merger. This material will consist of nuclides at or near the neutron dripline and having a proton count which may go as high as Z = 170. A zone of beta-decaying nuclides extending from the dripline to Uhq extends as low as A = 547, implying that the isotopes between Uhq 535 and Uhq 562 are likely to form in quantity (taking b+x*n decay into account), and that isotopes between Uhq 563 and Uhq 593 are possible, depending on where the dripline actually occurs. These nuclei will have millisecond-scale half-lives. Neutron capture is unlikely to produce more than an atom or two per star of nuclides heavier than A = 350. ATOMIC PROPERTIES Uhq is predicted to be a transition metal (d block), although it is never occurs in environments cool enough to have chemistry. Of more importance, its last (1s) ionization energy appears to be in the range 730 - 840 keV, meaning that bare nuclei are abundant only at temperatures above 6 gK. Electron configuration predicted for Uhq takes account of finite nuclear size, but nuclear shape is neither considered nor dismissed explicitly. Nuclear shape may have some effect on electron structure. REFERENCES 1. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011. 2. “The Highest Limiting Z in the Extended Periodic Table”; Y.K. Gambhir, A. Bhagwat, and M. Gupta; Journal of Physics G: Nuclear and Particle Physics.42 (12): 125105. DOI:10.1088/0954 3899/42/12/ 125105. 3. “Single Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region”; H. Koura and S. Chiba; Journal of the Physical Society of Japan; DOI 10.7566/JPSJ.82.014201; Jan. 2013. 4. "Neutron and Proton Drip Lines Using the Modified Bethe-Weizsacker Mass Formula; D.N. Basu et al; Int.J.Mod.Phys.; arXiv:nucl-th/0306061; url: https://arxiv.org/abs/nucl-th/0306061 5. "Magic Numbers of Ultraheavy Nuclei"; V. Yu Denisov; Physics of Atomic Nuclei, v.68, no. 7, pp 1133-1137; 2005. (02-14-20)